Known drainage systems of the auxiliary power unit (APU) of an aircraft comprise a mast for evacuating any liquid accumulated in the APU compartment to the atmosphere by gravity or pressure differential. Masts are supposed to release both small portions of fluid and large fluid leakages, so sizing is an issue given the different portions.
The known APU drain mast comprises a tube with or without a chamfered end. The drain masts comprise:
a first end configured for being connected to the source of liquid to be evacuated from the aircraft,
a second end configured for discharging the liquids to the atmosphere,
an internal surface configured for being exposed to the drained liquid, and
an external surface that can be exposed to the airflow surrounding the drain mast, that operate in favorable or adverse pressure gradients.
Each mast usually has a receptacle intended to receive the fluid from any source. This receptacle is able to lead the fluid towards the final drain mast. The shape of the mast makes the droplets of fluid run through the internal surface of the piping and finally they are released to the ambient. Therefore, the liquid film runs freely in the inner wall of the pipe.
In case of having small leakages, the droplets are not always released well and frequently once a droplet arrives at the open end of the mast, instead of being released, it adheres to the external surface of the drain mast going up to the fuselage, because when the aircraft is flying, the airflow can be powerful enough to cause the droplet to move to the rear external surface of the drain mast.
In addition to the previously stated drawbacks related to the improper release of the droplets, these masts are also a source of parasite drag to the aircraft.
The invention herein described tries to avoid these phenomena.